The signal level chosen to apply to Operational Transconductance Amplifier is a compromise between performance requirements. Noise, and DC offset encourage larger input signal voltages. Distortion encourages lower input voltage signals. In Operational Transconductance Amplifiers, noise, DC offset, distortion, and gain are all varying as a function of temperature to the extent that performance is often degraded. Newer Operational Transconductance Amplifiers employ some distortion cancellation techniques to allow larger levels of input signal voltage. The input stage of FIG. 1 was proposed by Okanobu in U.S. Pat. No. 4,965,528 issued Oct. 23, 1990. FIG. 1 shows this input stage uses transistor area scaling to apply equal and opposite DC offset voltages to two simple differential input stages. Both input stages are biased up with equal currents It1a and It1b. At the right magnitude of N, the distortion from transistors Qn1a and Qn1d are cancelled by the equal and opposite distortion from Qn1b and Qn1c. FIG. 1 adds a PNPs Qp1a and Qp1b to subtract the difference between the collector currents to produce a differential output current. The distortion of such an Operational Transconductance Amplifier architecture using a N value a 5 is shown in FIG. 2. For input signal expressed in terms of voltage, FIG. 2 shows that different levels of peak input voltage will have different levels of distortion at different temperatures. The distortion curves scale to absolute temperature. All Operational Transconductance Amplifiers without resistors, have this same relationship to temperature. This also applies to CMOS input stages which operate in the subthresshold region.
In addition to distortion, Operational Transconductance Amplifier see temperature variations in DC offset, noise, and gain. The transconductance transfer curve for the circuit of FIG. 1 is shown in FIG. 3. Gain is also has a direct relationship with absolute temperature. In most Operational Transconductance Amplifier applications, it is desirable for the voltage to current relationship of the transfer curve to come close to matching a perfect resistor. In gain or filter applications, an Operational Transconductance Amplifier stage will be functioning like an electrically controlled resistor. This invention teaches that the very steps that are needed to make an Operational Transconductance Amplifier behave like a perfect resistor also naturally set up the operation in the channel capacity format. The invention shows how to operate Operational Transconductance Amplifiers in a way that holds both impedances and performances to be constant regardless of temperature.